The invention relates to an optical component comprising a transmissive optical medium, and to a method of inducing a desired alteration of an optical property of such an optical component.
Optical lithography has become a key technology for the fabrication of electrical and optical integrated circuits. Since the smallness of such circuits is mainly determined by the imaging systems of the lithographic devices used in the fabrication process, considerable efforts have been spent on improving the resolution of these imaging systems.
One way to achieve higher imaging resolutions in such systems is to use shorter wavelengths. At present, research and development activities focus on devices that use UV light having a wavelength of 157 nm. Such extremely short wavelengths require transmissive optical components, such as lenses or compensators, that are made of very specific materials, for example calciumfluorid (CaF2) crystals, since conventional fused silica is almost opaque for such short wavelengths. However, the use of those materials poses new problems that are intrinsically coupled with their optical properties.
One of the particular problems that are encountered when CaF2 crystals are used is the physical effect of birefringence. The term birefringence refers to the dependence of the refractive index on the polarisation direction. If an unpolarized light beam impinges on a birefringing material, the incoming beam will be split up into two beams with different polarisations. The type of birefringence that is caused by the crystal structure as such is commonly referred to as intrinsic birefringence. Typically, this intrinsic birefringence is compensated by carefully choosing the crystal orientation of successive lenses within the imaging system.
Birefringence, however, may also be caused by mechanical stress within the crystal. Mechanical stress may be a result of poor crystal growth conditions or may be caused by lens holders or the like that exert a pressure on the crystal. Since the mechanical stress distribution within the crystals cannot be easily predicted in advance, compensation of stress induced birefringence is extremely difficult.
It is therefore an object of the invention to provide a method of inducing a desired alteration of an optical property, in particular of stress induced birefringence, of an optical component comprising a transmissive optical medium.
This object is solved, according to the present invention, in that such a method includes the step of exposing at least a region of the medium to a beam of ions having an energy of at least 100 keV so that the optical property is altered at least in a part of the region due to an interaction between the ions and the medium.
It is generally known in the art that optical components used in nuclear, space and high-energy physics severely suffer from radiation-induced performance degradation, see for example a paper by Johan van der Linden entitled xe2x80x9cResearchers compile radiation data basexe2x80x9d in Opto+Laser Europe (OLE), February 2002, issue 92, page 24-25. There it is described that exposure to particle radiation, such as proton and neutron beams, can cause displacement damage within optical materials, whereas exposure to electromagnetic radiation, such as gamma rays, will primarily induce defects resulting from ionisation.
The present invention, however, is based on the surprising finding that the exposure to particle radiation may be advantageously exploited for altering and adjusting optical properties of optical components if (heavy) ions with an energy of more than 100 kev per nucleon (100 kev/u), preferably an energy in the range between 90 MeV/u and 110 MeV/u, are used as projectiles. Since such ion beams lose, according to-the Bethe-Bloch-formula, almost all their energy in the last few millimeters before their motion is finally stopped in the material, the interaction of the incoming ions with the optical medium is also restricted to a very short distance along the propagation direction of the beam. It is thus possible to control and direct the interaction between the ions and the optical medium, and thus the alteration of optical properties, to a spatially confined region within the optical medium. Incidentally, this property of heavy ion beams is also exploited in the radiotherapy of deep seated tumors.
Preferably the optical component and the ion beam are moved relatively to each other in at least one direction that is distinct from the propagation direction of the beam. This allows to confine the region where the optical property is altered not only to a point or small sphere, but to a (curved) area within the optical medium. Since the penetration depth is, according to the Bethe-Bloch-formula, only a function of ion energy, any inclination of the ion beam with respect to the optical medium will result in a curved interaction area. Such an inclination of the ion beam may be accomplished by fast magnetic deflection as is principally known, for example, from TV sets. Any lateral translation between an ion beam source and the optical medium, as can be achieved by a x-y-handling system, for example, will result in a plane interaction area.
By varying the energy of the ions during the exposure of the region it is possible to control the penetration depth of the ions so that, if combined with a relative movement between the optical medium and the ion beam, a three dimensional region of arbitrary geometry may be xe2x80x9cwrittenxe2x80x9d by the ion beam so that the desired alteration of the optical properties can be spatially controlled in a very precise manner.
By varying the number of ions to which the region within the optical medium is exposed, it is possible to control the degree of the optical property alterations that are induced by the interactions between the ions and the optical medium.
The kind and strength of interaction is, of course, not only determined by the number of ions but also by the type of ions used for the exposure and also by the structural properties of the optical medium. For example, crystalline materials such as calciumfluoride are more sensible to ion beam exposure than amorphous media, for example silica glass. In general, it is possible to determine the optical property to be altered by carefully selecting the type of ions to which the optical medium is exposed. Ions of the type that is used for the ion beam may be already present within the medium. In the case of CaF2 lenses, for example, calcium or fluoride ions may be implanted into the crystal. However, it is also possible to implant different ions as a kind of dopant.
The ions may be chosen, for example, such that a change of the mechanical stress distribution is caused within the portion of the medium that has been exposed to the ion beam. Such a mechanical stress redistribution is mainly caused by the implantation of additional ions within the material. Also structural modifications along the penetration path of the ions within the medium have an influence on to the stress distribution within the material.
By carefully choosing the type of ions used and controlling the process of exposure it is thus possible either to achieve a reduction of the mechanical stressxe2x80x94and thus stress induced birefringencexe2x80x94within the optical medium, or to introduce additional stress within the optical medium. The latter may be useful, for example, if the optical component is designed as a correcting element that compensates birefringence of another optical component within the system.
By carefully choosing the ions it is also possible to selectively cause a local refractive index change within the optical medium. This effect may be used, for example, for transforming an originally spherical lens into an effectively xe2x80x9casphericalxe2x80x9d lens by altering the refractive index of the lens material by ion beam exposure in such a way that the processed lens has the same optical properties as an aspherical lens. This leads to a considerable cost reduction and is applicable not only in lithographic devices, but also in other technical fields in which optical components are used, for example photography or microscopy.
In another preferred embodiment the ions are optically absorptive at least for a range of transmitting light wavelengthes of the medium. Such implanted ions result in increased energy absorption and thereby to a local rise in temperature within the optical medium. Such optical elements may, for example, be used for compensating lens heating effects.